Conventionally, a large number of phosphor materials in which ions capable of emitting fluorescent light are doped into inorganic compound materials (hereinafter, referred simply as “phosphors”) have been known. Phosphors have the properties of emitting ultraviolet, visible and infrared light by action of external exciting means such as irradiation of electromagnetic waves (e.g., electron beams, X-rays, ultraviolet rays, visible light, etc.) or application of an electric field, and therefore are used in a large number of photoelectric transducers or photoelectric conversion devices. Examples thereof are light-emitting devices such as white light-emitting diodes (hereinafter, referred to as “white LED”), fluorescent lamps, electron beam tubes, plasma display panels, inorganic electroluminescent displays, and scintillators.
The phosphors will be described below. Inorganic compounds that can be phosphors by adding ions capable of emitting fluorescent light are known as a “phosphor host”, and the ions that emit fluorescent light by being doped to the phosphor host are known as “luminescent center”. A large number of inorganic compounds can be a phosphor host, and a typical compound among them is an inorganic oxide. A large number of inorganic oxides useful as a phosphor host have been found so far, and a large number of high efficiency phosphors have been put into practice. Specific examples of inorganic oxides include Y2O3, Zn2SiO4, LaPO4, BaMgAl10O17, Y3Al5O12, and GdMgB5O10 (e.g., non-patent reference 1: Fluorescent Handbook, edited by Phosphor Research Society, published on Dec. 25, 1987 by Ohmsha, Ltd, pages 192–240). In the research and development field of phosphors, there is a constant demand for development of novel phosphors with the increasing variety and high performance of phosphor application equipment.
Conventionally, a CaY2SnFe4O12 compound has been known as an inorganic oxide expressed by a chemical formula AB2CD4O12, where A is a metal ion that can be a bivalent ion, B and D are metal ions that can be trivalent ions, and C is a metal ion that can be a tetravalent (e.g., non-patent reference 2: Geller et al., J. Phys. Chem. Solids, 12, 111 (1959)). According to this reference, it is known that a CaY2SnFe4O12 compound has the crystal structure of garnet.
It is suggested that an inorganic oxide in which a part of or the entire Y of an inorganic oxide expressed by Y3Al5O12 is substituted with Mg, Ca, Sr, Ba Mn, Fe, Co, Cu, Ni, Zn, Cd, Pb or the like, and a part of or the entire Al is substituted with Si, Ge, Sn, Ti, Zr, Hf, Ru, or the like can be present as the inorganic oxide expressed by AB2CD4O12 having the garnet structure (e.g., non-patent reference 3: Baldassare Di bartolo Luminescence of Inorganic Solids Plenum Press pages 527–528). However, the inorganic oxide actually present is only CaY2SnFe4O12 (e.g., non-patent reference 4: PDF (Power Data File: 2000) of JCPDS-ICDD (Joint Committee on Powder Diffraction Standard International Center for Diffraction Data)), and there has been no report regarding inorganic oxides expressed by the above-described chemical formula other than CaY2SnFe4O12.
Conventionally, regarding phosphors, a phosphor expressed by a general formula v(R1−a−bCeaTbb)2O3.wDO.xAl2O3.ySiO2.zB2O3, where R is at least one of Y, La and Gd, D is at least one of Mg, Ca, Sr, Ba, and Zn, a, b, v, w, x, y and z are values satisfying 0<a+b≦1, v≠0, w≠0, 0≦x, 0≦y, 0≦z, x+y>0, y+z>0, and x+z>0, has been disclosed (patent reference 1: JP62-277488 A (page 1, Tables 1 to 4).
The patent reference 1 describes improvement of CeMgAl11O19:Tb3+ phosphor, which is well known as a green phosphor having a magnet plumbite structure, and other phosphors. This can be said for the following reason. In the phosphors disclosed in the examples of the patent reference 1, for example, the ratio (the number of atoms of the alkaline-earth metal element/the number of atoms of the rare earth element) of the alkaline-earth metal element (at least one of Mg, Ca, Sr, Ba, and Zn) to the rare earth element (at least one of Y, La and Gd) is in the range from 0.7 to 1.5, and there is no description regarding phosphors whose ratio is in the range from 0 to 0.5.
Apart from the above, regarding light-emitting devices (semiconductor light-emitting elements, illumination devices, or display devices), in recent years, semiconductor light-emitting elements/illumination devices/display devices in which a light-emitting diode (LED) and a phosphor are combined have attracted attention. A typical example thereof is a light-emitting device in which a blue LED using a gallium nitride host compound semiconductor material as an active layer is combined with a yellow light-emitting phosphor containing (Y, Gd)3Al5O12 inorganic oxide host material as the phosphor host and at least Ce3+ ions as the luminescent center (hereinafter, referred to as “YAG:Ce host phosphor”) (e.g., patent reference 2: Japanese Patent No. 2927279 (page 1), patent reference 3: Japanese Patent No. 3246386 (pages 1–6, Examples 1 to 5)). Such a light-emitting device in which a blue LED and a YAG:Ce host phosphor are combined can provide high intensity whitish light, for which there is a great demand for illumination and display, so that in recent years, the market is growing rapidly. In the light-emitting device in which a blue LED and a YAG:Ce host phosphor are combined, the bluish light emitted from the blue LED by allowing current to flow is absorbed by the YAG:Ce host phosphor, and the phosphor is excited with the bluish light, and converted to yellowish light with a high conversion efficiency. The mixture of the bluish light and the yellowish light can provide white light.
Recently, the YAG:Ce host phosphor also has been improved. Especially for use in illumination, the YAG:Ce host phosphor containing a large amount of a red light-emitting component is required, and for example, a phosphor in which Pr3+ ions are coactivated has been developed (patent reference 4:JP 2001-192655 (page 1, FIG. 1)).
Another proposal is application and development of a phosphor different from the YAG:Ce host phosphor that is excited with bluish light, and emits yellowish light (defined as including a wide range of yellow-greenish, yellowish, and orangish light).
For example, there are the following applications of the phosphors that are different from the YAG:Ce host phosphor: a phosphor using (Zn, Cd)S as the phosphor host has been applied to an electron beam tube (e.g., patent reference 5: JP 10-163535 A (page 3)); and a (Ba, Sr)2SiO4:Eu2+ phosphor has been examined to be used for a fluorescent lamp (e.g., patent reference 6: WO 02/054503A1). On the other hand, almost no development of the phosphors different from the YAG:Ce host phosphor has been performed, partly because it requires significant effort.
On the other hand, the patent reference 3 discloses a phosphor in which a part of Y of a Y3Al5O12:Ce phosphor is substituted with either one of Lu, Sc, La, Gd, and Sm and/or a part of Al is substituted with In, B, Tl, and Ga, and Si or Si and at least one selected from the group consisting of Ba, Sr, Mg, Ca and Zn are contained, and a light-emitting diode using the same.
However, the invention disclosed in the patent reference 3 basically is a phosphor that is an improvement of the YAG:Ce host phosphor. If the YAG:Ce host phosphor contains at least one element selected from the group consisting of Ba, Sr, Mg, Ca and Zn, and/or Si, a higher intensity LED lamp can be obtained.
Regarding the content of the above elements, it is preferable that at least one selected from the group consisting of Ba, Sr, Mg, Ca and Zn is contained in a ratio of 0.01 to 10.0%, and that Si is contained in a ratio of 0.001 to 5.0% in the specification of the patent reference 3. In other words, it is suggested that this phosphor contains these elements, not as the main component elements constituting the phosphor, but as impurity components that are doped in a small amount into the phosphor. In the examples, evidently, the YAG:Ce host phosphors in which the above-described elements are doped in a trace/small amount are described.
Furthermore, it is proposed that a phosphor substance used in the invention should be a host of a cerium-activated yttrium aluminum oxide host phosphor substance, and that specific examples are a phosphor having YAlO3, Y3Al5O12, or Y4Al2O9 as the phosphor host (generally called YAG host phosphors) and a mixture of phosphors having the above-described compounds as the phosphor host.
As described above, inorganic oxides expressed by the chemical formula AB2CD4O12 are mostly CaY2Fe4SnO12 having the crystal structure of garnet, and inorganic oxides having other compositions, in particular, such a composition that can be a phosphor host, have not been proposed. CaY2SnFe4O12 is an inorganic oxide that hardly serves as the phosphor host of a high efficient phosphor. For this reason, in the field of the material development of phosphors, no phosphor using a compound expressed by the chemical formula AB2CD4O12 as the phosphor host has been known.
On the other hand, a blue exciting yellow phosphor other than the YAG:Ce host phosphor for a light-emitting device (e.g., white LED) in which the blue LED and a blue exciting yellow phosphor other than the YAG:Ce host phosphor are combined has a fatal problem in that the luminescent efficiency under the excitation of blue light is at least 10% lower than that of the YAG:Ce host phosphor, and has a secondary problem in that there is only a little of a red light-emitting component, which is required especially when it is used for illumination.
At the same time, a light-emitting device in which the blue LED and a blue exciting yellow phosphor other than the YAG:Ce host phosphor are combined has a problem in that the light-emitting intensity is low, the intensity of a red light-emitting component is low, and this light-emitting is inferior to a device in which the YAG:Ce host phosphor is used.